The superfamily of pentameric ligand-gated ion channels (pLGICs) is one of the four superfamilies of synaptic ionotropic receptors present in animals; the other three are the excitatory glutamate receptors, the ATP-gated cation channels (P2X), and the acid-sensing ion channels (ASIC). Postsynaptic pLGICs mediate fast synaptic transmission, whereas presynaptic pLGICs modulate the release of other neurotransmitters. In addition, more recently, some pLGICs have been proposed to be involved in non-neuronal phenomena with the most compelling evidence suggesting a role for the ?7 nicotinic acetylcholine receptor in inflammation. Importantly, pLGICs are the target of therapeutic drugs (such as benzodiazepines and anesthetics) and recreational drugs (such as nicotine), and their malfunction is often associated with neurological disease?including neuropathic pain in diabetic patients, congenital epilepsy, schizophrenia and Alzheimer's disease. Much is known about the structure and function of these ion channels. However, progress in rational drug design?undoubtedly, a most intriguing and potentially rewarding application of our basic knowledge?has lagged far behind in part because our understanding of how structure gives rise to function in pLGICs remains incomplete. Here, we propose experimental work (electrophysiology, radioligand-binding assays, and direct structural approaches) and computational work (molecular and Brownian dynamics simulations, and electrostatic calculations) that will allow us to: 1) Understand the ?coupling? between ligand-binding and gating. We will challenge the prevailing view that the extracellular domain and the transmembrane domain form functionally autonomous units that need to be ?coupled? for the channel to function as a whole; 2) Determine the structure of these channels in functionally well-defined states and elucidate the effect of the membrane on their conformational free-energy landscapes; and 3) Characterize the impact of side-chain conformation at the selectivity filter on cation-versus-anion selectivity. Collectively, these three aims cover the three most fundamental aspects of ligand-gated ion channels, namely, ligand binding, gating/desensitization, and ion conduction/charge selectivity.